The present invention relates to a method of backing up data stored in a storage unit provided in a computer system. More particularly, the present invention relates to a method of backing up data from a disk system into a storage medium for a backup without having to interrupt I/O processing of an application program running on a computer.
A typical computer system needs to back up data stored in a storage medium such as a disk system for protecting the data from being broken by some unfavorable factors such as system failure, human inadvertent mistake, and disasters. In general, an ordinary user backs up the data at night once a day or a week by interrupting an online service. In recent days, however, a request for an uninterruptible online service is rising for the purpose of globalizing an enterprise or enhancing a customer's satisfaction level. For this purpose, a time to be spared for a data backup is made shorter and shorter.
As a technology for coping with such a request, there is known a technology disclosed in the white paper “Legato NetWorker for EMC Symmetric Installation and Administration Guide” of the Legato, Ltd. disclosed in a website “www.legato.com” on the Internet. The technology disclosed therein is arranged to provide a pair of disk drives in a disk system. In the ordinary online service, the data is duplicated and stored in these two disk drives. When the data stored in one disk drive (corresponding to a user volume) is updated by the online service, the updated content is reflected on the other disk drive (corresponding to a secondary volume). In backing up the data, in response to an instruction given from a host computer, the system is operated to write the data that is not reflected on the disk drive and held in a cache memory (the data is called dirty data), onto a disk drive, split a pair of disk drives into each one, and avoid reflection of the updated data in the user volume onto the secondary volume. Then, the host computer for performing the online service operates to continue the processing with the user volume. The secondary volume stores the data at the time of splitting the pair of disk drives. This data is copied onto a backup storage unit such as a magnetic tape device by means of another computer, for the purpose of backing up the data. Upon completion of the backup processing, the host computer operates to issue a re-synchronizing command so that it may return the user volume and the secondary volume to the duplex state.
This backup method makes it possible to obtain a consistent backup of the data because all the data is frozen at a time when all the dirty data has been reflected onto the primary volume. Further, the split of the pair state is completed for quite a short time, so that no interruption of the online processing is required. The data (frozen data) held in the secondary volume at the time of the pair split is called snapshot.
As another method of obtaining the snapshot, there are known a technology disclosed in the white paper “Celestra Architecture for Serverless Backups” and “Celestra Block Copy Interface Specification” of the Legato, Ltd. disclosed in the website “www.iguard.com” on the Internet. The technologies disclosed therein utilize a command for copying data from one volume to another volume or from a volume to a data device. This command is called a CCOPY command. In advance of obtaining a backup, the host computer operates to flush the dirty data and then write the data into a user volume. Then, the host computer uses the CCOPY command to copy the data from the user volume into the work volume. When the application program running on the host computer operates to write the data onto an area where no data is copied of the working volume, the copy program operates to detect it and keep the writing process waiting until the copy of the area is completed. The foregoing processing makes it possible to obtain the data at the time of starting the process of obtaining a backup process, on the working volume. After that, the host computer operates to use the CCOPY command for backing up the data from the working volume into the tape device.